Various types of motor current control systems for electric motors have been proposed in which the current for the motor is supplied by batteries. One such application is in electrically driven vehicles in which current from a secondary battery is supplied to an electric motor in pulse form. The duty cycle of the current supply is controlled by a thyristor, the switching state of which is controlled, in turn, by a control unit. The control unit receives input information in dependence on various operating parameters including, for example, an operator controlled motor controller to regulate the speed of the motor, temperature, charge state of the battery, or the like. To prevent overloading of the motor, it is necessary that the motor be supplied with current only if a command pulse is supplied from the motor control unit. Associating motor current and command pulses can be achieved by monitoring motor voltage or motor current.
It is customary to connect a free-wheeling or bypass diode in parallel to the motor so that inductive current can continue to flow even during pulse gaps from the pulse control unit. U.S. Pat. No. 3,689,818 discloses a control system for an electric power motor, with battery operation, in which a current flow signal is derived and this signal is compared with control pulses from the pulse source. Upon non-coincidence, the pulse control unit is disconnected if a current signal is sensed but no control pulse has commanded current flow. Current supply to the electric motor is then interrupted. Such continued current flow may arise, for example, due to a defect in the motor system, typically in the semiconductor switch which may be the aforementioned thyristor, permitting current to flow through the motor from the battery although no command signal has been received from the pulse control unit.